The invention relates to the laying of pipelines under water, especially at sea, from a vessel floating on the water.
The technique mainly used up to now to lay pipelines in the sea is that called xe2x80x9cS layingxe2x80x9d. This technique can be described briefly as follows. The pipe joints, generally 12 meters long, are transported from a port to a lay-barge by means of supply barges or supply vessels, and are loaded onto the deck of the lay-barge. Those pipe joints are added one after the other along a construction ramp, which is usually horizontal or in some cases has a small inclination of 5 or 10 degrees (this ramp being called the xe2x80x9cfiring linexe2x80x9d). On the firing line the operations necessary to complete the connection of the pipe joints are performed in several working stations in order to build a continuous pipeline. When a new joint is added, the barge moves forward and the pipeline, supported at the stern of the lay-barge by an inclined ramp (or floating stinger) curves over the stem of the barge down toward the seabed. The profile of the pipeline, from the lay-vessel to the seabed, is in the form of a long xe2x80x9cSxe2x80x9d (from which comes the term xe2x80x9cS layxe2x80x9d). The upper part of the profile is called the xe2x80x9cover-bendxe2x80x9d, and the lower part is called the xe2x80x9csag-bendxe2x80x9d. In order to reduce the stresses on the suspended part of the sealine on its way from the lay-vessel to the sea bottom, a constant tension is maintained on the pipeline by means of machines called tensioners. There is a maximum to the depth of water in which that method can be used. As the water depth increases, the tensioner pull necessary to maintain the pipe stress within acceptable values increases dramatically, and the horizontal bollard pull on the lay vessel increases correspondingly. A method to reduce the above-mentioned pulls consists of increasing the angle of descent of the sealine in to the sea. If the angle is close to vertical (called xe2x80x9cJ layxe2x80x9d) the necessary tension on the pipe is very close to the weight of a length of the sealine string equal to the water depth, and the horizontal component is close to zero. This method has the contrary limitation that there is a minimum to the depth of water in which it can be used. because the pipeline must have room to curve through about 90xc2x0 to lie on the seabed, and if the pipeline is too tightly curved it will be over-stressed.
It is an object of the present invention. considering all the above mentioned problems, to provide an apparatus able to lay pipeline in very deep waters as well as in relatively shallow waters, in conditions favourable for the integrity of the pipe and with high productivity.
The invention provides a pipe-laving vessel comprising: means for propelling the vessel during pipe-laving; means for assembling sections of pipe generally horizontally on the vessel to form longer lengths; a tower at the bow of the vessel, with respect to an intended direction of movement, pivotally mounted so that it can be angled forwards towards the top; means for raising a length of pipe from the deck to a position aligned with the tower; means for joining such a length of pipe to a pipeline being laid; tensioners arranged to grip such a pipeline and to lower it into the water while maintaining a desired tension in the pipeline; one or more clamps arranged to grip the pipeline below the tensioners; a lower ramp provided with rollers and arranged to guide the pipeline as it leaves the vessel; and means provided at a rear portion of the vessel for monitoring the pipe where it touches down on the water bottom.
The invention also provides a method of laying pipes from a vessel, comprising: providing a tower at the bow of the vessel, with respect to an intended direction of movement, pivotally mounted so that it can be angled forwards towards the top; repeatedly assembling sections of pipe in a generally horizontal position on the vessel to form a longer length, raising that length of pipe from the deck to a position aligned with the tower, joining that length of pipe to a pipeline being laid, and gripping the pipeline with tensioners and lowering it into the water while maintaining a desired tension in the pipeline; guiding the pipeline as it leaves the vessel using a lower ramp provided with rollers; monitoring the pipe from a rear portion of the vessel where it touches down on the water bottom; and gripping the pipeline with clamps below the tensioners during any interruptions in laying.
The mounting of the tower (firing line ramp) at the extreme bow of the vessel, and the accompanying inclination of the pipeline axis toward the front of the vessel, greatly facilitate the operation of passing the pipeline to a platform or other similar structure when constructing, for example, catenary risers. The end of the pipeline may be lowered free of the vessel using the abandonment and recovery winch, and then drawn up to the platform using one or more winches on the platform, without the need to pass the pipeline under the vessel or to manoeuvre the vessel out of a position between the end of the pipeline and the platform. Also, the pipeline, on its way to the bottom, runs under the vessel and the touchdown point is relatively close to a point directly below the stern. That makes it easy to monitor the touchdown and control the laying operation by means of a remotely operated vehicle (r.o.v.) based on the pipelaying vessel without the need for an extra survey vessel. The monitoring means may comprise means for operating an r.o.v. from the vessel, and the vessel may include such a remotely operated vehicle.
The means for propelling the vessel may comprise propellers driven by motors on the vessel. Instead, especially in relatively shallow water, the vessel man be propelled by capstans working on ropes attached to fixed anchors or the like.
Having the tower positioned in the bow, rather than amidships as has previously been proposed, allows a very large clear deck area for storing pipe containers and for assembling lengths of pipe. The assembling means advantageously comprises means for welding four sections of pipe to form a single length, preferably by welding them together in pairs and then joining the pairs. Thus, the vessel may be supplied with standard 12.2 meter joints, and raise 48.8 meter lengths to the tower.
The tower is advantageously pivotable between a vertical position and an angle of about 30xc2x0 forwards. The tower may also be capable of being lowered to a shallow angle for use as a launch ramp or stinger in S laying, but is preferably arranged to be dismounted to allow S laying from that end of the vessel. The end of the vessel that is the bow during J laying may then become the stem during S laying.
The raising means may comprise a cradle for supporting a length of pipe, pivotally mounted at or near the foot of the tower. Such a pivoted cradle can be raised and lowered by a rope from the tower. Preferably, the cradle is only approximately as long as the maximum length of pipe to be raised, in which case the pipe may be raised further up the tower by a vertical conveyor.
The joining means may comprise a first working station, equipped for actually welding the pipeline, and a second working station, one pipe length below the first, at which non-destructive testing and finishing of the join are carried out. One join may then be welded while the previous one is being tested, with a consequent increase in the rate of pipe laying. The second working station may be positioned between the tensioners and the clamps.
The tension machines may be a standard type previously used for S laying. The use of tensioners simplifies and speeds up the pipeline lowering operation, and allows the passage of anodes, buckle arrestors, etc. without problems.
The said clamps may comprise a fixed friction clamp and a movable friction clamp. Each of the clamps is preferably strong enough to support the maximum weight of pipeline that the vessel is capable of laying, even if the pipeline becomes flooded with water.
The lower ramp preferably comprises means for monitoring the load on the pipeline as it passes over the rollers, and is preferably of adjustable curvature so that the optimum curve can be imposed on the pipeline for any weight of pipeline in any depth of water that the vessel is capable of laying.
Preferably the lower ramp comprises a lower guide arrangement including a plurality of sets of guide rollers spaced apart along the path of the pipeline and defining the lateral limits of the path, the guide rollers being located such that they allow some bending of the pipeline as it passes through the lower guide arrangement. By allowing bending of the pipeline as it leaves the vessel and controlling the bending of the pipeline the likelihood of over-stressing the pipeline as it leaves the vessel is reduced. It becomes possible to arrange for the horizontal forces applied to the pipeline by the vessel during laying to be spread to between the various sets of guide rollers, so that the stresses introduced at any one set of guide rollers are reduced.
It will usually be preferable for the guide rollers to be of cylindrical shape, but other shapes can be adopted if desired and it is even possible for revolving tracks to be provided around some or all of the rollers, should that be desired for some reason.
Preferably the guide rollers of at least one set of rollers include rollers whose axes of rotation, in a plane perpendicular to the tower, are inclined to one another. In that case the guide rollers surround, at least to some extent, the pipeline and, thereby, allow for some variation between the alignment of the vessel and tower on the one hand and the vertical plane containing the undersea pipeline being laid on the other hand. Preferably the guide rollers of at least one set of rollers extend at least one quarter of a revolution around the path of the pipeline; that allows for a variation of about 90 degrees between the alignment of the vessel/tower on the one hand and the undersea pipeline on the other hand. More preferably the guide rollers of at least one set of rollers extend substantially all around the path of the pipeline; the guide arrangement is then operative for all orientations of the vessel relative to the undersea pipeline.
The lower guide arrangement is preferably of substantially trumpet shape flaring outwardly in the direction of travel of the pipeline during laying, the angle of flare increasing in the direction of travel of the pipeline during laying. In that case, whichever region of the guide arrangement is acting to guide the pipeline as it is being laid, has the effect of introducing a controlled bend into the pipeline and forces applied to the pipeline are spread between rollers of various sets.
Whilst it is within the scope of the invention for the guide rollers to fulfil some tensioning purpose, it is preferred that the guide rollers are freely rotatable so that substantially the only force applied to the pipeline by the guide rollers is a lateral force.
At least some of the rollers are preferably mounted for rotation on bearings that are directly or indirectly resiliently displaceable. It is advantageous to provide the resilience by allowing the axis of rotation of the roller to change resiliently rather than by, for example, providing a resilient roller structure so that the effective direction of the roller changes. It is possible for each roller to be mounted individually for resilient displacement but preferably a whole set of rollers is mounted on a structure that is resiliently displaceable.
The lower guide arrangement is of particular relevance to the laying of a relatively rigid pipeline rather than, for example, a pipeline that is so flexible it can be curved into reels for storage. Preferably the resistance of the bearings to resilient displacement is more than 100 kN/m, and more preferably more than 500 kN/m. In an embodiment of the invention described below, the resistance to displacement is of the order of 5000 kN/m in the case of certain higher sets of rollers and of the order of 1000 kN/m in the case of certain lower sets of rollers.
The bearings are preferably resiliently displaceable by a distance of more than 50 mm and preferably at least some of the bearings are resiliently displaceable by a distance of more than 100 mm. In an embodiment of the invention described below, the higher sets of rollers with a resistance to displacement of 5000 kN/m are able to be displaced 100 mm and the lower sets of rollers with a resistance to displacement of 1000 kN/m are able to be displaced 300 mm.
As will be appreciated, the amount of bending introduced into the pipeline during its passage through the lower guide arrangement may be small. One purpose of introducing the bend is to enable the horizontal forces between the guide rollers and the pipeline to be distributed evenly between the sets of rollers. In an embodiment of the invention described below, the amount of bending of the pipeline is of the order of 0.34 m per 10 m length of pipeline; in that embodiment the total force applied by the lower guide arrangement under normal conditions is about 1000 kN.
The inclination of the tower during use is preferably in the range of 45xc2x0 to 90xc2x0 to the horizontal.
Preferably three or more sets of guide rollers, and more preferably five or more sets of guide rollers are positioned along the path of the pipeline below sea level. Some sets of guide rollers are preferably also positioned along the path of the pipeline above sea level; bending of the pipeline is then able to begin above sea level and, in the case where the tower is pivotable, even above the axis of pivoting of the tower. Preferably the sets of rollers are spaced apart substantially evenly alone the path of the pipeline. The spacing along the path of the pipeline between adjacent sets of guide rollers is preferably in the range of 2 m to 15 m and more preferably in the range of 3 m to 10 m. In an embodiment of the invention described below, the spacing is about 5 m.
Preferably the vessel includes means for monitoring the forces applied to the pipeline by the rollers of the lower guide arrangement. Preferably a plurality of force monitoring means are associated with respective sets of guide rollers for monitoring the forces applied to the pipeline by the respective sets of guide rollers. The monitoring means may comprise a plurality of load cells.
Preferably, forces exerted on the pipeline by one or more of the guide rollers are monitored and the operation of the vessel adjusted in dependence upon the monitoring.
It will be appreciated that the pipe-laying vessel described above has many advantageous features that are most desirably combined together to enable the best results to be achieved, but which in many cases can be used separately from some or all of the other features, as will now be described more fully.
A first advantageous feature concerns the provision of the tower at the bow of the vessel such that pipeline being laid by the vessel is not enclosed by the vessel in the region of the hull of the vessel. Thus in a first advantageous aspect the invention provides a pipe-laying vessel comprising: means for propelling the vessel during pipe-laying; a tower at the bow of the vessel, with respect to an intended direction of movement, pivotally mounted so that it can be angled forwards towards the top; means for raising a length of pipe from a deck of the vessel to a position aligned with the tower; and means for joining such a length of pipe to a pipeline being laid; wherein the tower is placed at the extreme bow of the vessel. Similarly in the first advantageous aspect the invention also provides a method of laying pipes from a vessel, comprising; providing a tower at the bow of the vessel, with respect to an intended direction of movement, pivotally mounted so that it can be angled forwards towards the top; repeatedly raising lengths of pipe from a deck of the vessel to a position aligned with the tower; and joining each length of pipe in turn to a pipeline being laid; wherein the tower is at the extreme bow of the vessel, whereby pipeline being laid by the vessel is not enclosed by the vessel in the region of the hull of the vessel.
A second advantageous feature concerns the provision of two pipe joining stations along the tower spaced apart by one assembled length of pipe. Such an arrangement provides two distinct opportunities during pipe laying to make and/or inspect pipe joints with the result that less work has to be carried out on a given joint at either one of the two stations; thus the time for which the joint is required to remain stationary while a joint is made can be reduced. Thus in a second advantageous aspect the invention provides a pipe-laying vessel comprising: means for propelling the vessel during pipe-laying; a tower on the vessel; an upper pipe joining station at an upper location along the tower; means for raising a length of pipe from the deck to a position aligned with the tower and with the bottom of the length of pipe at substantially the level of the upper joining station; a lower pipe joining station at a lower location along the tower, separated from the upper joining station by substantially the length of one length of pipe; and means at the upper and/or lower joining stations for joining a length of pipe to a pipeline being laid. The raising means preferably comprises a first means for bringing the assembled lengths of pipe into a position generally aligned with the tower and a second means for lifting the assembled lengths of pipe along the tower. The second advantageous aspect of the invention also provides a method of laying pipes from a vessel, comprising: providing a tower on the vessel: repeatedly raising lengths of pipe from a deck of the vessel to a position aligned with the tower; and joining each length of pipe in turn to a pipeline being laid; wherein upper and lower pipe joining stations are provided on the tower the joining of the pipe is carried out at the upper and/or lower pipe joining stations and the pipe joining stations are separated by substantially the length of one length of pipe. Each raising step preferably comprises a first step of bringing the length of pipe into a position generally aligned with the tower and a second step of lifting the length of pipe along the tower.
A third advantageous feature of the invention concerns the provision of tensioners and clamps to maintain the desired grip on the pipeline. Thus in a third advantageous aspect the invention provides a pipe-laying vessel comprising: means for propelling the vessel during pipe-laying; a tower on the vessel; means for raising a length of pipe from a deck of the vessel to a position aligned with the tower; means for joining such a length of pipe to a pipeline being laid, tensioners arranged to grip such a pipeline and to lower it into the water while maintaining a desired tension in the pipeline; and one or more clamps arranged to grip the pipeline below the tensioners. In the third advantageous aspect the invention also provides a method of laying pipes from a vessel, comprising: providing a tower on the vessel; repeatedly raising lengths of pipe from a deck of the vessel to a position aligned with the tower; joining each length of pipe in turn to a pipeline being laid; gripping the pipeline with tensioners and lowering it into the water while maintaining a desired tension in the pipeline; and gripping the pipeline with one or more clamps below the tensioners during any interruptions in laying.
A fourth advantageous feature of the invention concerns the provision of a lower guide arrangement including a plurality of sets of guide rollers for guiding the pipeline after it has passed down the tower. Thus in a fourth advantageous aspect the invention provides a pipe-laying vessel including an upwardly extending tower assembly defining a path down which the pipe passes as a pipeline is being laid by the vessel, and a lower guide arrangement for guiding the pipeline after it has passed down the tower, the lower guide arrangement including a plurality of sets of guide rollers spaced apart along the path of the pipeline and defining the lateral limits of the path, the guide rollers being located such that they allow some bending of the pipeline as it passes through the lower guide arrangement. In the fourth advantageous aspect the invention also provides a method of laying a pipeline from a vessel, comprising lowering the pipeline down an upwardly extending tower assembly of the vessel and then through a lower guide arrangement, the lower guide arrangement including a plurality of sets of guide rollers spaced apart along the path of the pipeline and defining the lateral limits of the pipeline, the pipeline undergoing some bending as it passes through the lower guide arrangement.
It will be appreciated that each of the advantageous features of the invention described in the four paragraphs immediately above can be used all together, entirely separately from one another, or in any combination; they can of course also be used with any of the preferred features of the invention described earlier.
The vessel employed in any of the methods of the invention may be in any of the forms defined above.